In a typical automotive vehicle, a brake system includes brake pads disposed about a rotor mechanically coupled to a wheel and operated by a caliper assembly. During vehicle travel, the brake pads are spaced apart from the rotor to allow free rotation of the rotor, and thus the wheel. When stopping is desired, the caliper assembly clamps the brake pads against the rotor, creating friction that inhibits rotation of the rotor. In a conventional brake system, the assembly includes a hydraulic actuator coupled by a hydraulic line to a pedal that is depressed by the operator to engage the brake. When the pedal is depressed, the resulting increased hydraulic pressure advances a piston to apply the brake pads against the rotor.
The vehicle is also equipped with a park brake to prevent movement of the wheels when the vehicle is not in operation, such as when the vehicle is parked on an incline. A typical park brake comprises a cable linkage that mechanically connects a pedal depressed by an operator to a locking mechanism that secures the wheel. Care must be taken in routing the cable to assure smooth and reliable movement. Moreover, the pedal, linkage and locking mechanism add significant cost and weight to the vehicle. It has been proposed to use the hydraulic actuator as the park brake. However, when the vehicle is not in operation, as when the park brake is needed, the hydraulic pressure is reduced, and the actuator may not be or remain reliably engaged. As a result, to assure reliable operation, a separate mechanism generally provided for the park brake that does not rely solely upon or utilize the hydraulic actuator.
It has also been proposed to provide an electric park brake that includes a solenoid or other electromechanical device and secures the wheel independent of the hydraulic actuator. This allows the cable linkage to be replaced by a simple electrical wire. Also, it allows the park brake to be engaged by an electrical switch, as opposed to a pedal mechanism. However, in order to apply a force sufficient to clamp the brake pads against the rotor to prevent rolling of the vehicle, a relatively large electrical device is required. Also, the system must be designed to assure the park brake remains engaged despite a loss of electrical power, which might occur when the vehicle is not operating, for example, if the vehicle battery dies. These requirements increase the cost, size and complexity of an electro-mechanical park brake system.
Thus, there is a need for a brake actuator that is actuated hydraulically during vehicle operation to for purposes of stopping the vehicle, and that also may serve as a park brake by clamping the brake pads to secure the rotor, and thus the wheel, when the vehicle is not in operation. The hydraulic brake actuator used as a park brake must be reliable to maintain the brake pads engaged with the rotor with sufficient force to prevent wheel movement despite diminished hydraulic pressure when the vehicle is not operating. Moreover, it is desired that the park brake may be electrically activated, to provide a simple switch and wire connection to the operator, and, once engaged, must remain engaged even though the vehicle electrical system looses power.